Electronic semiconductor packages are well-known and are configured in several different ways. Typically, an electronic semiconductor package includes: a silicon chip (die) containing circuit elements; a substrate, for example, a printed circuit board (PCB); first level interconnects which connect the die and the PCB, i.e., wirebonds, Tape Automated Bonds (TAB) and Controlled Collapse Chip Connection (C4 or flip chip bonds); and second level interconnects, such as external metal pins or solder balls, which connect the substrate to printed wiring circuit cards. Substrates comprise ceramic or plastic materials depending of the particular application. Some semiconductor packages have encapsulant which coats the die and the first level interconnects for protection. A dam or stiffener ring may also be used to hold the encapsulant in place around the die and interconnects as it hardens.
A cross-sectional view of a typical cavity-up electronic semiconductor package is shown in FIG. 1. A semiconductor chip or die 1 is attached to a substrate 2 by die attach epoxy 1a. The die 1 electrically communicates with the traces 3 of the substrate 2 by bond wires 4. A stiffener ring 11 surrounds the die 1 and is attached also to the substrate 2 by epoxy 11a. Encapsulant 6 resides within the stiffener ring 11 and over the die 1 and wirebonds 4. A layer of epoxy 7 is spread over the encapsulant 6 and stiffener ring 11 and a heat sink 8 is attached thereto by the epoxy 7. Solder balls 9 attach the substrate 2 to a printed wiring circuit card, not shown.
During operation, energy is lost in the form of heat which builds up in the electronic semiconductor packages. As junction temperatures increase, reliability and speed are sacrificed. Because failure processes accelerate with increased temperature, the life of each junction becomes shorter as the temperature increases. It is generally well-known to provide means for dissipating heat from the electronic semiconductor package. It is particularly important to dissipate heat from electronic packages with semiconductor devices which operate at high speeds and high power levels.
The path of heat dissipation is considered in two parts: junction-to-case path and case-to-ambient path. The junction-to-case path usually conducts heat directly from the die to a package surface through an encapsulant epoxy. Then the heat is convected from the semiconductor package case to a surrounding ambient (case-to-ambient path) either by natural convection or forced convection. The case-to-ambient heat transfer is enhanced by increasing surface area exposed to ambient air. This is accomplished by employing heat sinks which have many configurations. Fluids such as air, water, etc. are passed over the heat sink to exchange heat from the heat sink surface to the fluid or ambient.
Because the heat dissipation effectiveness of a heat sink is dependent on the thermal resistance of the path between the package and the heat sink, the means of attaching the heat sink to the package is critically important. Typically, heat sinks are constructed from a high thermal conductivity material, such as copper, aluminum or high thermal conductivity plastic. They are attached to the packages with thermally conductive adhesive or epoxy, such as "LOCTITE-384" produced by Loctite Corp. However, if an adhesive is used to attach the heat sink, the heat sink is difficult to detach.
Detachable heat sinks are desirable in applications where it is necessary to remove damaged or defective packages. For example, packages which comprise solder balls as second level interconnects, referred to as "ball grid array" (BGA) packages, may only be removed from a wiring board by heating the solder balls to a melting temperature of the solder (reflow temperature). If the package has a non-detachable heat sink, more heat is required to melt the solder balls since the heat sink absorbs most of the heat provided due to high thermal capacitance of the heat sink material. The high intensity heat necessary to melt the solder balls causes damage, in many cases, to surrounding components on the wiring card. A detachable heat sink, however, provides easy access to the defective package by removing the heat sinks before providing sufficient heat to melt the solder balls. Therefore, less heat is required, compared to a non-detachable heat sink package, to melt the solder balls.
Some packages have heat sinks which are attached with screws. The screws extend from the heat sink into the substrate. These packages require a sufficiently thick substrate to support the screws, which in many applications is undesirable.
Alternatively, as shown in FIGS. 2a and 2b, a clip attachment device is shown for attaching a heat sink to an electronic semiconductor package. FIG. 2b is a cross-sectional view of the embodiments shown in FIG. 2a across line A--A. The clip 10 extends from one side of the semiconductor chip package to the other. In particular, the ends of the clip 10 curve around the edges of the package to engage the underside of the substrate 2. However, in this configuration, the clip 10 interferes with the solder balls 9 so that fewer input/output solder balls are available for use. Further, as shown in FIGS. 3a and 3b, in packages which employ a dam ring 5, the encapsulant 6 forms a mound over the die. FIG. 3b is a cross-sectional view of the embodiment shown in FIG. 3a across line B--B. These are called "glob-top" packages. The heat sink 8 only contacts the package at the top of the encapsulant 6 in the center of the package. The ends of the clip pull the edges of the substrate in the opposite direction to the force exerted by the heat sink 8 on the encapsulant 6. This generates a significant bending moment in the substrate.
Regardless of whether a package employs a stiffener or a dam ring, this method of clipping adds additional stress to the package substrate which warps the semiconductor package. This warpage is most significant in "glob-top" packages. Further, the clip, which wraps around and engages the edge of the substrate, interferes with the second level interconnects. This interference is particularly detrimental in the case of a ball grid array package.
Therefore, there is a need for a detachable heat sink attachment device which does not generate additional warpage of the package or interfere with the second level interconnects.